FIGS. 10 and 11 are views showing a conventional image sensor. FIG. 10 is a sectional view showing a structure of a general contact-type image sensor. Meanwhile, FIG. 11 is a f ront view of the contact type image sensor shown in FIG. 10 as viewed in the direction of the arrow. In FIGS. 10 and 11, 1 is a contact-type image sensor (hereinafter, referred to as image sensor), 2 is a document, 3 is a glass plate positioned in the document scanning plane, 4 is a sensor frame, 5 is a light source, 6 is a rod-lens array, 7 is a sensor IC, 8 is a sensor board, 9 is a L-form plate, 10 is a screw hole, 11 is a screw, 12 is a light shielding spacer and 13 is a cover provided on both sides of the sensor frame.
The parts structuring the contact type image sensor are assembled on the sensor frame 4. The sensor frame 4 is structured by two sensor frames 4A, 4B. The light source 5 is a line light source and uses a fluorescent tube lamp, light-emitting diode or the like. The image sensor 1 shown in FIG. 10 has two light sources 5 but one is satisfactory. The rod lens array 6 is a rod lens array for 1:1 imaging. This rod lens array 6 is fixed, without giving a gap, in the plane of contact between the sensor frames 4A and 4B and the rod lens array 6 by using a double-sided tape or adhesive. The sensor board 8 is arranged with the sensor ICs 7 in a line form. Meanwhile, the light shielding spacer 12, for preventing the sensor ICs 7 from being exposed to external light, is provided on the sensor board 8. The sensor board 8 is supported on the L-form plate 9 having a section in an L-form. The L-form plate 9 is fixed on the sensor frame 4A by using the screw hole 10 and screw 11. The diameter of the screw hole 10 is formed greater than the diameter of the screw 11.
Next, explanation is made on the operation of the image sensor 1. The light from the light source 5 passes through the glass plate 3 to illuminate evenly the reading surface of a document 2. The light reflected from the reading surface of the document 2 passes through the glass plate 3 and rod lens array 6 and focuses on the sensor IC 7. The reflection light from the document 2 reaching the sensor IC 7 contains the light/dark visual information characterizing the reading surface of the document 2. The sensor IC 7 detects the light/dark information and stores corresponding electric charges, and outputs the charge information to the outside through a circuit formed on the sensor board 8. The image sensor 1 prior to its use is adjusted for proper focusing so that the reflection light from the document 2 can be correctly focused on the sensor IC 7.
Although different depending on a kind of the image input/output apparatus, the distance x between the reading surface of a document 2 transported through not-shown rollers and the glass plate 3 is set to about 0.4 mm–1.0 mm. Focus adjustment is made on the premise that the distance between the reading surface of the document 2 and the glass plate 3 is kept at nearly 0.4 mm–1.0 mm. The screw hole 10, provided in the L-form plate 9 mounted with the sensor ICs 7 and sensor board 8, has a diameter formed greater than the diameter of the screw 11. Consequently, within its range the L-form plate 9 can adjust the distance between the sensor IC 7 and the rod lens array 6. Focus adjustment is made by tightening the screw 11 to move the sensor IC 7 to the position where the reflection light from the document 2 is correctly focused on the sensor IC 7. The distance required for the reflection light from the document 2 to be properly focused on the sensor IC 7, is referred to as the focal length.
The explained contact type image sensor accommodates the glass plate 3, lens such as a rod lens array 6, light source 5, sensor IC 7 and an image reading mechanism such as the sensor IC board 8 properly within a housing such as a cover 4. The image sensor described above reads a document of large size, such as A0 or A1 size. The lengthwise size is 1 meter or somewhat smaller for reading out A0 size and 0.7 meter or somewhat smaller for reading out A1 size. On the other hand, the thickness is as small as nearly 40 mm. Differently from the sensor for a size-reducing optical system requiring a predetermined focal length between a focusing sensor and a document, the contact type image sensor can be conspicuously reduced in focus light path length hence having a merit that the image input/output apparatus can be greatly reduced in size.
In the meanwhile, the image sensor for adoption in an image input/output apparatus for inputting and outputting a document in a large size such as A0 or A1 size has a lengthwise dimensions of 0.7–1.0 meter or somewhat smaller relative to the thickness of nearly 40 mm. Consequently, there is a problem that the contact type image sensor is insufficient in lengthwise rigidity and readily bends at the center by its own weight. FIGS. 12 and 13 are explanatory views showing a contact type image sensor bent at the lengthwise center. FIG. 12 is a view showing the state that the glass plate 3 of the image sensor 1 is attached facing in an upward direction. FIG. 13 is a view showing the state that the glass plate 3 of the image sensor 1 is attached facing in a downward direction.
As shown in FIG. 12, in the case that the image sensor 1 attached with the glass plate 3 in the upward direction is bent in its longitudinal center by its own weight by −Δ1 in a concave form with respect to the document 2, and so the distance between the document reading surface at a lengthwise center and the glass plate becomes greater than the prescribed value (0.4–1.0 mm). Meanwhile, as shown in FIG. 13, where the image sensor 1 attached with the glass plate 3 in a downward direction is bent in its longitudinal center by Δ1 in a convex form with respect to the document 2, the distance between the document reading surface at the center and the glass plate becomes smaller than the prescribed value (0.4–1.0 mm). Namely, if the image sensor is bent by its own weight, the focal length between the document reading surface and the sensor IC varies. Due to this, the reflection light from the document reading surface is not correctly focused on the sensor IC thus causing deterioration of the resolution of the image read.
The present invention has been made in order to resolve the above explained problem.